Along with the exhaustion of natural wood resources and protection of national natural woods, artificial wood will become the main resources for national and international timber markets. Artificial wood includes fir, masson pine, larch, cottonwood and paulownia. They are featured in fast-growing speed, high yield quantity and short harvesting cycle. However, they have limited applications because of some innate defects caused by fast-growing speed such as low wood quality, low density and surface hardness, poor corrosion resistance and climate resistance, and easy to be distorted or cracked.
Wood functions improvement is an effective approach to change physical and chemical characteristics of artificial wood, whose density, surface hardness, corrosion resistance and dimensional stability improve to a large extent after process. Then they can be used for solid wood floor boards, furniture and construction.
One of the methods to improve mechanical property of wood section material is compression.
Many studies have been done on fast-growing trees such as cotton wood, southern pine and masson pine due to their loose texture and large anisotropy and a number of successful domestic and international experiences can be used for reference.
However, the compressed wood rebounds seriously, especially under the conditions of water.
In order to eliminate the defects of the compressed wood, steam or resin is often used to fix the compressed wood to prevent rebound. For example, in 1996, Mrs. Fang Guizhen et al. used melamine-formaldehyde (MF) resin with different concentration of low molecular weight as the cross-linking agent to soak populus ussuriensis with 47% for ASE and 36% for MEE. The samples after processed by cross-linking agent with 10% concentration, compressed and soaked in room temperature, they were able to maintain their shape. It is still applicable to use cross-linking agent with 17.5% and 25% concentration and soak in boiling water. In 1997, Mrs. Fang Gunzhen studied the action mechanism between populus ussuriensis and MF cross-linking agent. In 1998, Mrs. Fang Guizhen et al. used PF performed polymer with different concentration to process populus ussuriensis and compress the populus ussuriensis along its transverse striation in heating process. It was found that the samples after the process of 10% PF performed polymer, ASE reached to 60% and MEE reached to 52%. It is able to maintain the shape no matter it was in room temperature or boiling water. In 1998, Mrs. Fang Guizhen et al. used 1, 2, 3, 4-butanetetracarboxylic acid (BKA) as the cross-linking agent and NaH2PO2 as the accelerant to process populus ussuriensis, which is then compressed under constant temperature 150° C. Mrs. Fang Guizhen et al. used PF resin with different concentration of low molecular weight to process populus ussuriensis to improve its dimensional stability and mechanical strength. In 2000, Mrs. Fang Guizhen et al. conducted a research to use low chromaticity phenolic resin with low molecular weight to soak the wood and use phenolic resin with low molecular weight to improve the property of populus ussuriensis.
The existing surface-compressed solid wood is to soak the surface of the processed wood into the water for five to six hours and use microwave to heat the wood after some amount of water is infiltrated. Then it is compressed on the heating device and dried afterwards to fix the compressed product. However, after the wood is soaked by water, its surface contains large amount of water. When the water vaporizes, the wood surface would quickly contract to produce a large internal stress. Therefore, the surface is easy to crack or even rebounded because the internal stress of the surface is not balanced and fully solidified in the process of compression and drying.
Because the above method uses chemical agent to process the wood, it is inevitable to produce water gas/water and noise. The finished product has poor dimensional stability/corrosion resistance/climate resistance, low service life/yield quantity and easy to be distorted or cracked.
Chinese patent document (CN101603623A) publicized the patent of “a type of surface-reinforced solid wood section material/floor board and their manufacturing method”. The manufacturing method includes: (1) dry the log; (2) compress the log in a hot-press with temperature from 210 to 250° C.; (3) maintain the temperature of the compressed log for 20 to 60 minutes; and (4) control the moisture content of the log within 6 to 9%. However, the wood is easy to shrink and crack in drying by using above process with large wood consumption, low yield quantity (60% to 70%), dark color and burning smell. The finished floor boards can only be used in northern China because of the dry weather. In southern China, the boards would be distorted with low corrosion resistance (grade III) and poor anti-corrosion ability.
Chinese patent document (CN101214675A) publicized the patent of “Wood Reinforcement Method by Hot-pressed Carbonization”, in which (1) dry the wood in a drying kiln according to its density and control the moisture content within 3 to 17%; (2) polish the wood; (3) Put the polished wood in a hot-press with temperature from 160 to 260° C. for carbonization. Control the compression ratio within 5 to 50% and maintain the temperature for 10 to 240 minutes; cool the carbonized wood to below 80%; (4) place the wood in natural environment or a temperature & humidity test chamber and control the moisture content in 5 to 10% according to the purpose of the wood. By using above method, the wood is easy to be cracked during the carbonization process with low yield quantity (50% to 60%), low corrosion resistance (below grade III), poor dimensional stability. Meanwhile the color of the wood would become darker due to excessive carbonization with burning smell.
Chinese patent document (CN101486212A) publicized the patent of “Manufacturing Method of Carbonized Three-layer Solid Laminated Floor Board (cottonwood)”. The preparation of the surface material includes: cut cottonwood into panels and process them into thin boards according to compression ratio (30%, 40%, 50% and 60%) with thickness from 2-4 mm and moisture content from 20 to 40% after drying and polishing. Compress the boards in a press and use the pressure based on the required compression ratio under 70 to 110° C. Carbonize the compressed boards with certain pressure or in a dedicated clamp under 190 to 220° C. for 1.5-5 hours. As the carbonization device does not have ventilation, cool the thin boards to 40 to 60° C. under certain pressure after carbonization. Take out the boards and use wide-belt sander to remove the outer surface with dark color. The thickness of the finished boards would be at 2 to 4 mm. By using above method, because of large moisture content of cottonwood (at saturation point of fiber), the shrinkage ratio and surplus stress would be large under 70-110° C. and the wood is easy to crack during the carbonization under 190 to 220° C. As the compression ratio is big, the wood will be compressed in a whole piece, therefore the wood consumption becomes large. The ventilation holes in carbonization device will make the surface of the wood having uneven points. After polishing, the hardness of the boards where those points are present will decrease. If there is no moisture content adjustment treatment, the wood will be distorted if it absorbs humidity. Consequently such process is not good enough to be widely applied.